THE suspected serial killer poisoning saline drips in a hospital could still be stalking the wards, police believe.

Three people have already died and yesterday it was revealed that a fourth – a man in his 40s – is fighting for life.

Sixty detectives are now investigating the deaths at Stepping Hill Hospital in Stockport, Greater Manchester. Security has been stepped up and medics have been ordered to work in pairs.

Chris Burke, chief executive of the local NHS Trust, said: “This is a criminal act. We do not believe it could have been anticipated.”

The deaths of Tracey Arden, 44, George Keep, 84, and Arnold Lancaster, 71, are being linked to the contamination of saline solution.

Insulin was found in a batch of 36 saline doses in a storeroom close to Ward A1 of the hospital.

Mr Burke added: “I would understand why patients would be worried about coming into this hospital. But I can say that it is safe. We hope to stop further incidents like this.

“I can see no reason, at this time, why anybody should be concerned about previous deaths.

“We are working very closely with the police at what is an investigation in its early stages.” Greater Manchester Assistant Chief Constable Ian Hopkins said that “at the moment” the investigation is concentrating on 14 patients, including the three who had died.

He added: “When we have got a clear suspect is the time we will look at historic deaths.”

Officers have questioned a large number of people. They are awaiting the results of further tests on those who died to establish the cause.

Mr Hopkins said: “It is unlikely there will be very early arrests. We have spoken to 50 witnesses so far and that number continues to grow.

“Our investigation is concerned with people who work in or visit the hospital. We are not ruling out that it could be someone outside.

“Our thoughts continue to be very much with the families of Tracey Arden, George Keep and Arnold Lancaster. The anguish they must be feeling at this time will be terrible.

“We also recognise this is a very difficult time for all the staff here.”

Detectives are concentrating on staff and visitors to the two hospital wards involved – A1 and A3 – described as acute medical wards.

There is a “core” staff of medics on the wards but occasionally agency staff have worked there.

James Catania, medical director at Stepping Hill, said insulin was always kept in a refrigerator in a locked room.

Saline solutions are now also being locked away.

A single key holder has to be asked for access to locked drugs cabinets and names and times are noted each time drugs are given to patients.